Transport in Animals
Summary: Mammals have a double circulatory system consisting of the pulmonary circulation (heart → lungs → heart) and systemic circulation (heart → body → heart). The heart is a four-chambered muscular pump with valves ensuring unidirectional blood flow. The pacemaker (SAN) controls the heartbeat. Blood is composed of red blood cells, white blood cells, platelets, and plasma. Arteries, veins, and capillaries are adapted for their specific roles. Tags: igcse biology circulatory-system heart blood blood-vessels double-circulation Created: 2026-07-16 Last Updated: 2026-07-16
1. The Need for a Circulatory System
Large, multicellular organisms (e.g. mammals) cannot rely on diffusion alone to supply all cells because:
- The surface area to volume ratio is too small
- Most cells are too far from the external environment
- A circulatory system transports oxygen, nutrients, hormones, and waste products; it also distributes heat
The circulatory system consists of the heart (pump), blood vessels (tubes), and blood (transport medium).
2. Double Circulation
Mammals have a double circulatory system — blood passes through the heart twice in one complete circuit of the body.
| Circuit | Pathway | Function |
|---|---|---|
| Pulmonary circulation | Right ventricle → pulmonary artery → lungs (capillaries around alveoli) → pulmonary vein → left atrium | Pumps deoxygenated blood to the lungs to pick up oxygen (gas exchange) and release CO2. Blood pressure is relatively low to protect delicate alveolar capillaries |
| Systemic circulation | Left ventricle → aorta → body tissues (capillaries in organs) → vena cava → right atrium | Pumps oxygenated blood to all body tissues to deliver oxygen and nutrients, and collect CO2 and waste. Blood pressure is high to reach all body parts |
Advantages of double circulation:
- Blood can be pumped to the lungs at a lower pressure (protects delicate capillaries) and to the body at a higher pressure (ensures efficient delivery)
- Oxygenated and deoxygenated blood are kept separate — more efficient oxygen delivery to tissues
- Higher blood pressure in systemic circuit allows for a higher metabolic rate
3. Heart Structure
The heart is a muscular organ located in the thoracic cavity, made primarily of cardiac muscle (which does not fatigue).
Chambers
| Chamber | Function |
|---|---|
| Right atrium | Receives deoxygenated blood from the body via the vena cava (superior and inferior) |
| Right ventricle | Pumps deoxygenated blood to the lungs via the pulmonary artery |
| Left atrium | Receives oxygenated blood from the lungs via the pulmonary vein |
| Left ventricle | Pumps oxygenated blood to the body via the aorta |
Key structural features:
- The left ventricle has a thicker muscular wall than the right ventricle — it needs to pump blood at high pressure around the entire body (systemic circulation), whereas the right ventricle only pumps to the nearby lungs
- The septum (a wall of muscle) separates the left and right sides — prevents mixing of oxygenated and deoxygenated blood
Valves
Valves in the heart ensure unidirectional (one-way) flow of blood and prevent backflow.
| Valve | Location | Function |
|---|---|---|
| Tricuspid valve (right atrioventricular valve) | Between right atrium and right ventricle | Prevents backflow from ventricle to atrium when ventricle contracts |
| Bicuspid / mitral valve (left atrioventricular valve) | Between left atrium and left ventricle | Prevents backflow from ventricle to atrium when ventricle contracts |
| Semilunar valves (pulmonary and aortic) | At the base of the pulmonary artery and aorta | Prevent backflow from arteries into ventricles when ventricles relax |
How valves work:
- Valves are flaps of tissue that open when pressure is higher on one side and close when pressure is higher on the other
- Tendinous cords (chordae tendineae) anchor the AV valve flaps to the ventricle walls — prevent the valves from turning inside out (prolapsing) when the ventricle contracts
Major Blood Vessels Connected to the Heart
| Vessel | Connected to | Carries | Direction |
|---|---|---|---|
| Vena cava (superior + inferior) | Right atrium | Deoxygenated blood | Body → heart |
| Pulmonary artery | Right ventricle | Deoxygenated blood | Heart → lungs |
| Pulmonary vein | Left atrium | Oxygenated blood | Lungs → heart |
| Aorta | Left ventricle | Oxygenated blood | Heart → body |
The pulmonary artery is the only artery that carries deoxygenated blood. The pulmonary vein is the only vein that carries oxygenated blood. Arteries are defined by direction of flow (away from heart), not by oxygen content.
4. The Heartbeat Cycle
The heartbeat (cardiac cycle) consists of three phases:
| Phase | Atria | Ventricles | Valves | What Happens |
|---|---|---|---|---|
| Atrial systole (contraction) | Contract | Relaxed | AV valves open, semilunar valves closed | Blood is forced from atria into ventricles |
| Ventricular systole (contraction) | Relaxed | Contract | AV valves closed (prevent backflow), semilunar valves open | Blood is forced from ventricles into pulmonary artery and aorta. AV valves closing produces the “lub” heart sound |
| Diastole (relaxation) | Relaxed | Relaxed | AV valves open, semilunar valves closed (prevent backflow from arteries) | Atria fill with blood from vena cava and pulmonary vein; blood flows passively into ventricles. Semilunar valves closing produces the “dub” heart sound |
Heart rate (pulse) = number of cardiac cycles per minute. Average resting: ~70 bpm. Measured at the wrist (radial artery) or neck (carotid artery).
5. The Pacemaker — Sinoatrial Node (SAN)
The heartbeat is myogenic — the heart muscle contracts on its own, without stimulation from nerves.
The sinoatrial node (SAN) is the natural pacemaker:
- Located in the wall of the right atrium
- A group of specialised muscle cells that produce electrical impulses at a regular rate
- The impulses spread across the atria, causing them to contract (atrial systole)
- The impulses are delayed slightly at the atrioventricular node (AVN) before travelling down the Bundle of His and Purkinje fibres to the ventricles — this delay allows the atria to finish contracting before the ventricles start
- The ventricles then contract from the bottom upwards (ventricular systole)
- The SAN is influenced by nerves (sympathetic increases rate, parasympathetic decreases rate) and hormones (e.g. adrenaline increases heart rate)
6. Coronary Heart Disease
Coronary arteries branch off the aorta and supply the heart muscle itself with oxygenated blood. The heart does not use the blood flowing through its chambers for its own oxygen supply.
Coronary heart disease (CHD):
- Caused by the build-up of fatty plaques (atheroma) inside the coronary arteries — this narrows the lumen and reduces blood flow
- Reduced blood flow → reduced oxygen supply to heart muscle → angina (chest pain during exertion)
- Complete blockage by a blood clot (thrombus) in a narrowed coronary artery → heart attack (myocardial infarction) — part of the heart muscle dies from lack of oxygen
Risk factors:
- Diet high in saturated fats and cholesterol
- Smoking (nicotine constricts blood vessels; carbon monoxide reduces oxygen-carrying capacity of blood)
- Lack of exercise
- Obesity
- High blood pressure (hypertension)
- Genetic predisposition
- Stress
Prevention:
- Healthy diet (low saturated fat, high fibre)
- Regular exercise
- Not smoking
- Maintaining a healthy body weight
7. Blood Vessels
Arteries
Function: Carry blood away from the heart (usually oxygenated; exception: pulmonary artery).
| Feature | How It Relates to Function |
|---|---|
| Thick muscle layer in the wall | Can withstand high blood pressure from heart’s pumping; can constrict/dilate to regulate blood flow |
| Thick elastic layer | Stretches when blood is pumped in (during systole) and recoils (during diastole) — smooths out pressure surges and maintains blood pressure between heartbeats (felt as pulse) |
| Narrow lumen (relative to wall thickness) | Maintains high pressure |
| No valves (except semilunar valves at base of pulmonary artery and aorta) | Blood is under continuous high pressure so backflow is not a problem |
Veins
Function: Carry blood towards the heart (usually deoxygenated; exception: pulmonary vein).
| Feature | How It Relates to Function |
|---|---|
| Thin muscle and elastic layers | Blood is at low pressure, so a thick muscular wall is not needed |
| Wide lumen (relative to wall thickness) | Reduces resistance to blood flow under low pressure |
| Valves along the length | Prevent backflow of blood — ensures blood flows in one direction (towards heart) despite low pressure and the pull of gravity |
| Blood flow assisted by skeletal muscle contraction | When skeletal muscles contract (e.g. during walking), they squeeze adjacent veins and push blood towards the heart |
Capillaries
Function: Exchange of substances between blood and tissue cells (oxygen, CO2, nutrients, waste).
| Feature | How It Relates to Function |
|---|---|
| Wall is one cell thick (just the endothelium) | Very short diffusion distance for rapid exchange |
| Very narrow lumen (~diameter of a red blood cell) | RBCs pass through in single file — every cell is close to the capillary wall; slows blood flow, giving more time for exchange |
| Permeable walls (pores/gaps between endothelial cells) | Allows dissolved substances to pass through easily |
| Extensive branching network | Provides a large total surface area for exchange |
8. Blood Components
| Component | Structure | Function |
|---|---|---|
| Red blood cells (erythrocytes) | Biconcave disc shape; no nucleus (in mammals); contains haemoglobin (red, iron-containing protein) | Transport oxygen from lungs to tissues. Haemoglobin + oxygen ←> oxyhaemoglobin (reversible reaction). Biconcave shape → large surface area for O2 diffusion; no nucleus → more room for haemoglobin |
| White blood cells — Phagocytes | Lobed nucleus, can change shape | Engulf and digest pathogens by phagocytosis. Non-specific — attack any foreign organism |
| White blood cells — Lymphocytes | Large spherical nucleus, small cytoplasm | Produce antibodies specific to antigens on pathogens; some become memory cells for long-term immunity |
| Platelets | Small fragments of larger cells (megakaryocytes); no nucleus | Involved in blood clotting: release chemicals that convert soluble fibrinogen into insoluble fibrin (mesh) at wounds — prevents blood loss and entry of pathogens |
| Plasma | Pale yellow liquid (~90% water, ~10% dissolved substances) | Transport medium for: blood cells, dissolved nutrients (glucose, amino acids), hormones, antibodies, carbon dioxide (as hydrogencarbonate ions, HCO3-), urea, heat, plasma proteins (fibrinogen, albumin) |
9. Tissue Fluid and Lymph
Tissue fluid (interstitial fluid):
- At the arterial end of a capillary, blood pressure is higher than osmotic pressure → plasma (minus large proteins and cells) is forced out through capillary pores → forms tissue fluid
- Tissue fluid bathes cells — allows exchange of substances between blood and cells by diffusion
- At the venous end of a capillary, blood pressure has dropped and is lower than osmotic pressure → most tissue fluid is reabsorbed by osmosis
- The small amount of tissue fluid not reabsorbed drains into lymphatic vessels → becomes lymph
Lymph:
- Similar composition to tissue fluid
- Transported through lymph vessels back to the blood (drains into veins near the neck)
- Lymph nodes along the vessels contain lymphocytes — filter pathogens and foreign material
Sources
- BBC Bitesize GCSE Biology — Circulatory system / Cardiovascular system, BBC (free educational resource)
- OpenStax Biology 2e — Ch. 40 The Circulatory System, Rice University (free, CC BY 4.0)
- Cambridge IGCSE Biology 0610 — Syllabus 9: Transport in animals, Cambridge Assessment International Education
- CK-12 Biology for High School — Cardiovascular System, CK-12 Foundation (free, CC BY-NC 3.0)
Related Notes
- Cell Structure and Organisation — Red blood cells as specialised cells; cardiac muscle
- Movement Into and Out of Cells — Diffusion of O2, CO2, and nutrients at capillaries; formation of tissue fluid
- Human Nutrition — Transport of absorbed nutrients from ileum to tissues
- Gas Exchange in Humans — Gas exchange at alveoli; transport of oxygen and carbon dioxide in blood
- Diseases and Immunity — Role of white blood cells (phagocytes, lymphocytes) in immune defence
- IGCSE-Bio-Index — Full IGCSE Biology index
Common Misconceptions
| Misconception | Reality |
|---|---|
| ”Arteries always carry oxygenated blood” | Arteries are defined by direction of flow (away from heart), not oxygen content. The pulmonary artery carries deoxygenated blood to the lungs |
| ”Veins always carry deoxygenated blood” | Veins carry blood towards the heart. The pulmonary vein carries oxygenated blood from the lungs to the heart |
| ”The heart receives its oxygen from the blood flowing through its chambers” | The heart muscle is supplied by the coronary arteries, not by the blood in the chambers — the wall is too thick for diffusion |
| ”The right ventricle wall is as thick as the left” | The left ventricle has a much thicker wall because it must pump blood around the entire body (high pressure); the right ventricle only pumps to the lungs (lower pressure) |
| “Red blood cells have a nucleus” | Mature mammalian red blood cells have no nucleus — they lose it during development to make more space for haemoglobin |
| ”Veins don’t have any valves; only the heart does” | Veins have pocket valves along their length to prevent backflow of blood under low pressure |
| ”Diastole means the heart stops” | During diastole, the heart muscle is relaxing (not contracting), but it is still filling with blood — the heart never fully “stops" |
| "Blood is just a red liquid” | Blood is a tissue — it contains red cells, white cells, platelets, and plasma, each with specific functions |